I hope your tomatoes are growing as well as ours. Our three plants have produced dozens of cherry, red pear and beefsteak tomatoes. They are still small and green but all they need is more warm weather and regular watering to grow and ripen into the delicious red fruits of summer. But alas, there are a host of diseases that can still infect our tomato plants. Fusarium and Verticillium are two soil borne fungi that can affect the leaves of tomato plants, sometimes resulting in complete defoliation of the plant.
Fusarium caused by the fungus Fusarium oxysporum f. lycopersici begins with the yellowing and wilting of the oldest leaves of the plant. The fungus thrives in warmer weather (optimal soil temp 82°F) and is more severe in acidic soil. The pathogen most often enters through root wounds caused by cultivation or by nematode feeding. Usually the disease starts on one side of the plant and moves to the younger leaves, often resulting in the death of the plant. This disease can occur anytime during the growing season but if the plant is infected early on, it can result in little or no fruit produced. Plants infected later often produce normal fruit near the bottom, but small or no fruit at the top of the plant.
Symptoms of Verticillium, caused by Verticillium albo-atrum and V. dahliae, usually appear in mid-August when the lowest leaves begin to yellow, wilt and fall off. (It is common in cooler daytime temperatures of 68° to 75° which seems less likely to occur in August these days.) The fruit is usually well established by this time, but if the plant is substantially defoliated the fruit may be damaged by over exposure to the sun (sun-scald). Fusarium and Verticillium may occur year after year because these fungi can overwinter in the soil.
In spite of its name, Early Blight generally shows up in mid to late summer. It is a fungus, Alternaria solani, that is quite common in Minnesota. It causes spotting in leaves, stems and starting at the stem end of the fruit. It can also result in the defoliation of the plant. It starts with brown to black leathery spots on the lowest leaves, which then turn yellow, shrivel and drop, once again exposing the fruit to sunscald. Late Blight (Photophthora infestans) usually shows up in late summer to early autumn. Ideal conditions for this fungus are moderately warm days (60º -70º) and cool nights (50º-60º), and high humidity or over-abundant moisture. During these weather conditions an entire plant can be infected within days. The fungus is inactive during dry periods. Water-soaked leaf lesions and white thread like material on the underside of the leaf are the fungus symptoms. It can also infect the fruits showing as olive-colored spots sometimes covering the whole fruit.
Septoria Leaf Spot (Septoria lycopersici) is a fungus whose spores can survive the winter on plant debris or on some perennial weeds, such as nightshade. In the spring the spores can be passed to new plants either by wind or leaf splash. It can infect many parts of a tomato plant resulting in leaf spots and stem lesions. First symptoms are round yellow spots on the lower leaves shortly after the fruit is set. These spots grow and turn gray or black, and eventually small black fruiting bodies produce spores, which cause infection further up the plant. Heavily infected leaves turn yellow, then brown and drop.
Many of these tomato diseases have similar symptoms making diagnosis of the particular disease difficult. The best way to prevent and control all these diseases (and others that we have not mentioned) is good cultural practices.
When possible buy/plant disease resistant plants. You can buy plants that have resistance to Fusarium and Verticillium bred into them. You can identify these by the letters V and F after the cultivar’s name on the label or tag. You may also find plants with an N or T after the cultivar indicating a resistance to nematodes and tobacco mosaic virus. Best practices for preventing and/or controlling diseases in your tomato plants are:
- No overhead watering. Water your plants at ground level to prevent soil that may contain spores or fungi from splashing onto the leaves. Also too much water or humidity on the leaves can contribute to the development of Late Blight. Water at the base of the plants in the morning before the temperature and humidity rises.
- Mulch the soil to prevent soil from splashing onto the leaf. Mulching will also keep the soil temperatures down and prevent weed germination.
- If a disease does develop on your tomato plants take precautions to prevent the disease from spreading or reoccurring on next year’s plants
- Remove all diseased plant material from the plant before it hits the ground. This prevents the fungus or spores from entering the soil.
- Cage or stake your plants to provide good air circulation and keep the leaves and fruit off the ground.
- Clean all soil and plant debris off of your tools every time you work with the infected plants.
- Don’t grow your tomatoes in the same spot more than once every three or four years.
With good cultural practices, plenty of warm sunny days and plenty of moisture I wish you an abundant crop of tomatoes this August.
“The man who has planted a garden feels that he has done something good for the world” – Vita Sackville-West